It is well known that measurements of resistivity of subsurface formations provide useful information to engineers and geologists engaged in hydrocarbon exploration and production and other fields, such as mining. Resistivity logging is well-known in the industry. In some cases, it is performed by inducing a current to flow in the formation (and other conductive materials proximate the logging tools) and then selectively measuring the current distribution. Open-hole resistivity logging is a well-developed art, wherein the drill pipe and bit are removed from a well being drilled, and an open-hole resistivity logging tool is lowered into the wellbore and used to obtain the desired information.
Furthermore, measurement-while-drilling (MWD, also known as logging-while-drilling) systems have been developed, whereby resistivity measurements may be obtained while the drill pipe is in the hole. MWD systems permit log information, such as resistivity, to be measured in a geologic formation very soon after the formation is penetrated by the drill bit. This provides substantially "real-time" information that (a) is obtained before the formation is substantially altered by inflow of drilling fluids or other factors, and (b) may be used by the driller to control the drilling operation, for example by steering the bit so as to penetrate (or so as not to penetrate) a selected formation, which can be detected by the logging apparatus. These systems typically include transmitters and sensors disposed in or on sections of drill pipe that are located near the drill bit.
A drillstring typically comprises a bit, drill collars, and drill pipe. The lowest part of the drillstring is made up of collars. The collars are heavy walled pipe that provide weight on the bit and strength to resist buckling under their own weight. The drill pipe is thinner walled, and it is kept in tension to prevent buckling. The collars may have radial projections called stabilizers. Short drill collars, which may be adapted for specialized functions, are called "subs," and references herein to drill collars are intended to include associated subs, depending on the context of the reference, as will be appreciated by those skilled in the art.
In some prior art MWD systems, for example as described in U.S. Pat. No. 5,235,285, a toroidal transmitting transformer is built into a drill collar and it creates a current field that flows through the drillstring and the formation. Sensors, which may be in the form of buttons, rings, or toroids, are mounted in the collar and positioned to measure the magnitude of the induced current field at selected locations. The electronic components that control the transmitter and that process the received signals are located in an annular chassis that is located within the drill collar, such that the entire instrument is contained within one piece of collar pipe, which includes a continuous annular channel to allow the drilling mud to flow through it. The system includes means for communicating the collected information to the surface. Several types of communication systems are well known in the art, including use of electrical or acoustic signals that are transmitted from a downhole transmitter to a receiver at the surface, and use of memory storage systems to record the data within the tool for retrieval when the tool is brought to the surface.
There are several problems associated with the described arrangement: (a) The system is complex, expensive and unwieldy, because all of the electronics and sensors are built into a piece of pipe that must be stout enough to support the weight placed on the bit; (b) the entire collar must be handled in order to repair any part of the system; and (c) if anything goes wrong with the system, the entire drillstring must be pulled out of the hole to gain access to it.
In U.S. Pat. No. 4,786,874, which is incorporated herein by reference, a different sort of MWD resistivity tool is described, wherein a pair of electrodes are positioned in an insulated jacket that is formed on the outside surface of a drill collar. Both electrodes are axially positioned on the same side of the collar, and they provide a directional resistivity measurement that can be used to indicate, while drilling, that the bit is approaching a boundary between high- and low-resistivity formations. The components of this system are contained in a drill collar, and an adequate annular fluid flow path is maintained to allow drilling mud to flow through the tool. This system suffers from the same deficiencies as the system discussed above, plus problems associated with the insulated jacket, which is likely to be made of a relatively soft material. The jacket, and the electrodes it supports, are exposed to hostile wellbore conditions during drilling and are likely to be severely eroded and damaged both by contact with the wellbore walls and by the flow of abrasive mud and cuttings up past the tool.
Some prior art systems have employed a cartridge or cartridge 42 is placed inside of the drilling pipe near the drill bit, the cartridge containing the electronic circuitry or instrumentation for the logging tool, or providing electrical connection between downhole components and surface equipment. The drill bit and portions of the down-hole assembly, which may be insulated, may serve as the logging electrodes. Spring contacts or brushes are typically used to provide electrical connections between such electrodes and the circuitry contained in the cartridge. In some systems, the cartridge is connected to the surface equipment by conductors, typically contained in an armored cable. See U.S. Pat. Nos. 2,596,390 and 2,650,067 (reel for cable at surface rotates with drill pipe), which are incorporated herein by reference. In other systems, the cartridge is positioned in the drillstring near the bit, and the collected data is transmitted to the surface by wireless means, such as by electrical signals (U.S. Pat. No. 2,364,957), by acoustic signals (U.S. Pat. No. 4,553,097) or by storing the data until the cartridge is retrieved from the wellbore (U.S. Pat. No. 3,293,542) (all referenced patents being incorporated herein by reference).